Spontaneous Pattern Formation of Colloidal Particles by Evaporation-Induced Self Assembly

自底向上的自组织方法有望制造由亚微米粒子组成的高阶图案表面。第一个被广泛研究的图案是条纹；然而，到目前为止，条纹图案的形成仅限于部分或完全疏水表面。相比之下，我们通过对流自组装技术成功地在强亲水性衬底上制备了定义良好的条纹图案。通过使用这种技术，只需将衬底悬浮在稀释的悬浮液中，就可以产生条纹图案，无需任何复杂的程序；条纹自发地平行于接触线排列。我们发现，无论是粘滑运动和脱湿，控制过程的疏水表面，是相关的这项技术；相反，粒子浓度的负反馈，由半月板的凹弯曲形状引起，控制着条纹的形成。然后我们考察了悬浮液中离子浓度的影响。在低浓度区域，条纹形成垂直于接触线，这可能是由于指法的半月板的尖端不稳定。另一方面，在高浓度区域形成垫片图案，这可能是因为离子在颗粒和衬底之间起润滑剂的作用。

Bottom-up self-organization approaches are promising for fabricating higher-order patterned surfaces composed of submicron particles. The first example among the patterns that have been extensively studied would be stripes; however, the formation of stripe patterns has so far been confined to partially or fully hydrophobic surfaces. By contrast, we have succeeded in preparing well-defined stripe patterns even on strongly hydrophilic substrates via a convective self-assembly technique. By using this technique, a stripe pattern was produced simply by suspending a substrate in a dilute suspension, without any complicated procedure; the stripes spontaneously aligned parallel to the contact line. We reveal that neither stick-slip motion nor dewetting, which control processes on hydrophobic surfaces, is relevant to this technique; instead, negative feedback of particle concentration, induced by a concavely curved shape of the meniscus, governs the stripe formation. Then we examined the effect of ion concentration in a suspension. In a low concentration region, stripes formed vertical to the contact line, which would be due to the fingering instability of the tip of the meniscus. On the other hand, a gasket pattern formed in a high concentration region, which would be because ions play a role as a lubricant between particles and a substrate.